1. Field of the Invention
The present invention relates to an apparatus for controlling operation of a hydraulic servo unit which may be used for the control of the speed reduction ratio of a continuously variable transmission.
2. Description of the Prior Art
There have heretofore been proposed various continuously variable transmissions in which the speed reduction ratio between input and output shafts is continuously variable. For example, Japanese laid-open patent publication No. 56-95722 discloses a continuously variable transmission for use on a motor vehicle, which comprises a hydraulic closed circuit composed of a fixed-displacement hydraulic pump and a variable-displacement hydraulic motor.
While the motor vehicle is running, the speed reduction ratio of the continuously variable transmission is controlled by a hydraulic servo unit that is controlled based on a throttle opening, a vehicle speed, etc. One known apparatus for controlling operation of such a hydraulic servo unit is disclosed in Japanese laid-open patent publication No. 1-108466, for example. In the disclosed apparatus, a first hydraulic pressure supply passage for supplying working oil under a predetermined hydraulic pressure is connected to a rod-side chamber in a cylinder serving as a hydraulic servo unit, and a second hydraulic pressure supply passage is connected to a head-side chamber of the cylinder. The second hydraulic pressure supply passage is connected to the first hydraulic pressure supply passage through a first solenoid-operated valve which can be opened and closed based on the duty cycle of a pulse signal applied to the solenoid thereof. The second hydraulic pressure supply passage is also connected to a drain circuit through a second solenoid-operated valve which can be opened and closed based on the duty cycle of a pulse signal applied to the solenoid thereof.
To reduce the speed reduction ratio of the continuously variable transmission, the first solenoid-operated valve is opened to supply working oil to the head-side chamber of the cylinder, and the second solenoid-operated valve is closed. To increase the speed reduction ratio, the second solenoid-operated valve is opened to allow working oil to flow out of the head-side chamber of the cylinder, and the first solenoid-operated valve is closed.
Each of the first and second solenoid-operated valves can increase or reduce the rate at which the working oil flows linearly in proportion to the increase or reduction in the duty cycle of the pulse signal insofar as the duty cycle falls within a range of 20 to 80%, for example, which is referred to as a "linearity control range".
If the duty cycle decreased below the linearity control range, then the solenoid-operated valves would operate incompletely, i.e., would start to be closed before the flow passage therethrough is not fully opened, i.e., its cross-sectional area does not reach a maximum area. The rate at which the working oil flows would not increase or decrease in proportion to the duty cycle, thus making it difficult to control the hydraulic servo unit for low-speed or inching operation accurately through the control of the duty cycle.
Even when the solenoid-operated valves are actuated with the same duty cycle, the rate at which the working oil flows varies depending on the temperature of the working oil. It is therefore difficult to control the hydraulic servo unit to operate slowly with stability at all dimes.
Higher vehicle running performance standards require that the speed reduction ratio of a continuously variable transmission be precisely controlled to meet running conditions that vary from time to time, even while the vehicle is running substantially steadily to meet such a requirement, the hydraulic servo unit needs to operate accurately and stably especially at very low-speed. However, inasmuch as the solenoid-operated valves cannot accurately be controlled with those duty cycles which are smaller than the linearity control range, and also since the oil temperature affects the working oil rate, it has heretofore been quite difficult to control the speed reduction ratio of the transmission.